Distortion correcting circuit and display device

ABSTRACT

A distortion correcting circuit for correcting vertically asymmetric distortion while achieving reduction in circuit scale and a display apparatus equipped with the circuit are provided. This distortion correcting circuit comprises a comparator circuit  10  which compares an amplitude of an vertical sawtooth wave Vsaw with a reference level Vref which is set to the amplitude of the vertical sawtooth wave corresponding to a center of the screen in a vertical sweeping direction, a multiplier circuit  11  which generates a parabolic wave of the difference between the vertical sawtooth wave and the reference level, a variable gain type amplifier circuit  12  which adjusts the output amplitude of the multiplier circuit  11  and a control circuit  13  which sets a first and second gains of the variable gain type amplifier circuit  12  based upon a result of the comparison in said comparator circuit when a result of the comparison in the comparator circuit  10  represents that the amplitude of the vertical sawtooth wave is lower, and equal to or higher than the reference level, respectively. A distortion correction signal Vsaw is output from the amplifier circuit  12.

FIELD OF THE INVENTION

The present invention relates to a display apparatus and in particularto a circuit for correcting pincushion distortion and a display devicecomprising the circuit.

BACKGROUND OF THE INVENTION

In a cathode ray tube (termed “CRT”) of a Television receiver or displayapparatus, as shown in FIG. 16a, while a deflection radius of a uniformmagnetic field for deflecting an electron beam 73 emitted from anelectron gun (cathode electrode 72) defines a spherical face, a screen(fluorescent surface) 74 is substantially planer so that the distancefrom the deflection center to the screen gets longer with the increaseof the deflection angle and the projection size becomes larger as araster image projected on the screen 74 moves toward the edges of thescreen 74. The raster image are extended most at the four corners wherethe deflection angle becomes the largest and are distorted in the formof a pincushion and hence this distortion is termed “pincushiondistortion”. As shown in FIG. 16b, the distortion in which the verticalline at opposing right and left edges on the screen are curved isreferred to as “E-W Raster pincushion distortion” or “Side pincushiondistortion” since it is the distortion at the right and left edges.

Correction of the left and right pincushion distortion has heretoforebeen achieved by causing a horizontal deflection current which isamplitude-modulated with a parabolic current in a vertical scanningperiod to flow through a horizontal deflection coil.

FIG. 17 is a block diagram showing an exemplary configuration of a CRTmonitor in which a deflection distortion such as a side pincushiondistortion is corrected. Referring to FIG. 17, the CRT monitor comprisesa vertical sawtooth wave oscillating circuit 207 which receives avertical synchronization signal Vsync output from a synchronizationseparating/synchronization input processing circuit 205 and generates avertical sweeping sawtooth wave (termed vertical sawtooth wave) and adeflection distortion correcting circuit 208 which receives the verticalsawtooth wave output from the vertical sawtooth oscillating circuit 207and generates a distortion correction signal Vo made up of a parabolicwave of one vertical sweeping period. The distortion correction signalVo which is generated by the deflection distortion correcting circuit208 is fed to a horizontal oscillating circuit 206 which receives ahorizontal synchronization signal Hsync and generates a horizontalsweeping sawtooth wave (horizontal drive signal). The horizontal drivesignal (the phase of which is controlled by the distortion correctionsignal) which is output from the horizontal oscillating circuit 206 isfed to a horizontal deflection output circuit 210. The horizontal drivesignal output from the horizontal oscillation circuit 206 and thedistortion correction signal Vo output from the deflection distortioncorrection circuit 208 are fed to a +B power supply circuit 209 whichboosts the +B voltage by a pulse amplification in a blanking periodduring which a high voltage is required. As is well known, one cycle ofthe sawtooth wave is made up of a scanning interval during which theelectron beam is scanned at a constant rate and a blanking intervalwhich is a period from the completion of the scanning in one directionto next scanning. The amplitude of the horizontal drive signal withinone vertical period is corrected by correcting +B voltage with thedistortion correction signal Vo, a frequency of which corresponds to onevertical period. The +B voltage is supplied by the +B power supplycircuit 209 to the horizontal deflection output circuit 210 whichprovides a deflection current which flows into a horizontal deflectioncoil. The horizontal deflecting current in which the amplitude of thehorizontal drive signal is changed in proportion with that of thedistortion correction signal is output from the horizontal deflectionoutput circuit 210. The synchronization deflection circuit is configuredas mentioned above. As an output circuit of a video signal (RGB signal),there exists an RGB preamplifier 201 which receives a input video signal(RGB input) and outputs pre-amplified signal to an RGB main amplifier202. The amplified output signal from the RGB main amplifier 202 is fedto an electron gun (not shown) of the CRT 203. In the CRT 203,deflection currents from horizontal and vertical deflection outputcircuits 210 and 211 are supplied to the horizontal and verticaldeflection coils 204, respectively and the electron beam which isemitted from the electron gun is deflected under the magnetic fieldsfrom deflection yokes (not shown) arranged on the neck of the CRT 203.

A slight variation in a mounting angle of the deflection yoke on theneck of the CRT 203 in a vertical direction causes a difference in adistortion wave form of a pincushion distortion, trapezoidal distortionand the like between the upper and lower areas of the screen, and hencea vertically asymmetrical distortion is generated.

Although the raster images at the upper and lower areas of the screen ofthe CRT have been corrected by the same amount so that the distortionsare not readily visible, implementation of a feature to independentlycorrect the side pincushion distortion at upper and lower areas of thescreen has been demanded with a recent increase in a size and a planarstructure of the screen of CRT.

Apparatuses for correcting the side pincushion distortion or verticallyasymmetric distortion are disclosed in, for example, Japanese PatentKokai Publication JP-A-6-334887, JP-A-5-308538 and JP-A-11-313222.

Among of them, in JP-A-6-334887 is disclosed a configuration of a cornerpincushion distortion correcting wave generating circuit which iscapable of independently correcting the distortions at corners in theupper and lower levels of the screen. As shown in FIG. 18, thecorrecting wave generating circuit comprises a lower corner partextracting differential comparator circuit 102 which receives a sawtoothwave signal 101 (vertical sawtooth wave signal) having a period of onevertical interval and extracts (slices) a part of the waveformcorresponding to the lower area of the screen in the sawtooth wavesignal 101; a multiplier circuit 105 for multiplying the extractedwaveform; and an amplitude adjusting circuit (gain control circuit) 107for adjusting the amplitude of the corrected wave, whereby theabove-mentioned circuits correct the corner distortion in the lower areaof the screen. The correction wave generating circuit further comprisesan upper corner part extracting differential comparator circuit 103which receives the sawtooth wave signal 101 and extracts a part of thewaveform corresponding to the upper area of the screen in the sawtoothwave signal 101; a multiplier circuit 106 for multiplying the extractedwaveform; and an amplitude adjusting circuit (gain control circuit) 108for adjusting the amplitude of the corrected wave, whereby the circuitscorrect the corner distortion at the upper area of the screen. Thecorrecting wave generating circuit further includes an amplitudeadjusting circuit (gain control circuit) for adjusting the amplitude ofthe signal after the lower corrected wave is added to the uppercorrected wave in an adder 109. A corner slice point bias circuit 104provides reference bias voltages used in the lower and upper cornerimage extracting differential comparator circuits 102 and 103respectively for extracting the lower and upper corner parts in imagesto the lower and upper corner extracting differential comparatorcircuits 102 and 103, respectively.

In JP-A-5-308538 is disclosed a left and right side pincushiondistortion correcting apparatus for correcting the side pincushiondistortion wherein a current flowing through the horizontal deflectioncoil of a horizontal output circuit is modulated in a vertical period bya parabolic voltage generated by integrating the vertical sawtooth wavevoltage and wherein the apparatus comprises a correction section set upunit for forming a selection signal for each correction section which isobtained by dividing each of a vertical synchronization period withreference to the vertical synchronization signal or horizontalsynchronization signal and a correction quantity adjusting unit whichselects a correction adjusting voltage for each correction section inresponse to a selection signal of each correction section forsuperimposing the correction adjusting voltage of each correctionsection to the parabolic wave voltage. With this apparatus, thecorrection section is obtained by dividing the vertical synchronizationperiod, the correction quantity adjusting voltage is given to eachsection, the correction quantity adjusting voltage in each correctionsection is put on the parabolic voltage, and parabolic wave voltage isgenerated at an integration circuit by integrating the vertical sawtoothwave voltage so that the central position of the amplitude of thevertical sawtooth wave matches the bottom of the parabolic wave voltageand the correction interval is preset with reference to the verticalsynchronization signal and the correction section is set up withreference to the vertical synchronization signal.

In JP-A-11-313222, is disclosed an image distortion correcting apparatuswhich reduces distortion which is asymmetric in upper and lower parts ofa screen. With this apparatus, an output of the side pin correctingcircuit is a parabolic wave plus a third power wave (S-shape wave). Theamplitude of the parabolic wave of the output signal is changed byvarying a side pin control voltage, and hence a pincushion distortionwhich is symmetric in left and right sides of the screen can becorrected. The amplitude of the third power wave is changed by varyingan S-shape distortion control voltage, and hence vertical S-shapedistortion which is asymmetric in upper and lower parts of a screen canbe corrected. The correction quantity in the third power wave isautomatically changed depending upon the vertical size of the screen andvertical position by using a vertical sawtooth wave includinginformation on the vertical size of the screen and the verticalposition. With this apparatus, the distortion adjustment operation iscomplicated because the operation adjusts two signals (the parabolicwave and the 3rd power wave) for the distortion of the whole screen tobe minimized.

SUMMARY OF THE DISCLOSURE

As mentioned in the above, the apparatus which is disclosed inJP-A-6-334887 has two systems each comprising a differential comparatorcircuit, multiplier circuit and amplitude adjusting circuit for each ofthe upper and lower parts of a screen in order to control independentlythe upper and lower parts of the screen. Accordingly, the apparatus hasa problem that the circuit scale increases. Parts in a vertical sawtoothwave corresponding to a lower part (lower corner) and an upper part(upper corner) of the screen are respectively extracted by thedifferential comparator circuits 102 and 103 and are respectivelymultiplied by the multiplier circuits 105 and 106. If there arevariations in the offsets of two differential comparator circuits 102and 103, the variations in the offsets are also multiplied by themultiplier circuits 105 and 106. The correction signal which is obtainedby adding the lower waveform to the upper waveform may have an offset(step) due to the difference between the offsets in the upper and loweradjusting circuits. When the gains of two amplitude adjusting circuits107 and 108 are changed, the offset voltage are changed, which maychange the raster image size.

It is an object of the present invention to provide a distortioncorrecting circuit and a display apparatus in which a distortion isindividually corrected for each one in the upper and lower parts of ascreen while reducing a circuit scale.

Another object of the present invention is to provide a distortioncorrecting circuit and a display apparatus which is capable ofcorrecting properly distortions in a lower and upper areas of a screenwhich are asymmetric even when a center of a vertical sawtooth wave doesnot correspond to a center of the screen.

At least one of the above mentioned objects is accomplished by adistortion correcting circuit in accordance with one aspect of thepresent invention which comprises: comparing means for comparing anamplitude of a sawtooth wave signal used for a vertical sweep (termed“vertical sawtooth wave”) with one predetermined reference level or aplurality of predetermined reference levels to detect which part of ascreen in a vertical sweeping direction a currently sweeping position islocated; amplifying means for adjusting an amplitude of a sidepincushion distortion correction signal generated from said verticalsawtooth wave; and control means for individually setting a gain foreach of partial regions of the screen to said amplifying means on thebasis of a detection result by said comparing means.

In accordance with another aspect of the present invention is provided adistortion correcting circuit-comprising comparing means for comparingan amplitude of a vertical sawtooth wave with a predetermined referencelevel to detect which of an upper and lower areas of a screen a currentsweeping position is located at; and control means for individuallysetting for the upper and lower areas of the screen the gain ofamplifying means which amplifies the amplitude of a side pincushiondistortion correction signal generated from said vertical sawtooth wavebased upon a result of the detection conducted by said comparing means.

More specifically, a distortion correcting circuit in accordance withanother aspect of the present invention comprises: a comparator circuitwhich receives a sawtooth wave signal used for a vertical sweep (termed“vertical sawtooth wave”) and compares an amplitude of said verticalsawtooth wave in a scanning period with one reference level or aplurality of different reference levels, each of which is setrespectively to an amplitude value of the vertical sawtooth wavecorresponding to each predetermined position of the screen in a verticalsweeping direction; a variable gain type amplifier circuit which adjustsan amplitude of an output signal from a multiplier circuit forgenerating a parabolic wave; and a control circuit which alternativelyselects one of a plurality of gains to provide the selected gain to saidvariable gain type amplifier circuit on the basis of a comparison resultby said comparator circuit depending upon which of regions divided bysaid reference level the amplitude of said vertical sawtooth wave islocated, wherein a distortion correction signal is output from saidvariable gain type amplifier circuit.

A distortion correcting circuit in accordance with another aspect of thepresent invention comprises: a comparator circuit which receives asawtooth wave signal used for a vertical sweep (termed “verticalsawtooth wave”) and compares the voltage of said vertical sawtooth wavein a scanning period with a reference level or a plurality of referencelevels, each of which is set to an amplitude of the vertical sawtoothwave corresponding to a predetermined vertical position of a screen; avariable gain type amplifier circuit which amplifies said verticalsawtooth wave; a control circuit which selects one of plurality ofpredetermined gains depending upon which of regions divided by saidreference level the amplitude of said vertical sawtooth wave is locatedon the basis of a comparison result by said comparator circuit andprovides the gain selected to the variable gain type amplifier circuit;and a multiplier circuit which generates a parabolic wave from an outputsignal output from said variable gain type amplifier circuit to outputsaid parabolic wave as a distortion correction signal

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only the preferred embodiment of theinvention is shown and described, simply by way of illustration of thebest mode contemplated of carrying out this invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, all without departing from the invention. Accordingly, thedrawing and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of the first embodiment ofthe present invention;

FIG. 2 is a diagram showing the configuration of the second embodimentof the present invention;

FIG. 3 is a diagram showing a modification of the second embodiment ofthe present invention;

FIG. 4 is a circuit diagram showing an exemplary multiplier circuit;

FIG. 5 is a diagram showing the configuration of the third embodiment ofthe present invention;

FIG. 6 is a diagram showing the configuration of the fourth embodimentof the present invention;

FIG. 7 is a circuit diagram showing an example of the detailedconfiguration of the first embodiment of the present invention;

FIG. 8 is a circuit diagram showing an example of the detailedconfiguration of the second embodiment of the present invention;

FIGS. 9a to 9 d are views for explaining the principle of the operationof the embodiment of the present invention;

FIGS. 10a to 10 d are views for explaining the principle of theoperation of the embodiment of the present invention;

FIGS. 11a and 11 b are graphs for explaining the offset of thedistortion correction signal in the embodiments of the presentinvention;

FIGS. 12a and 12 b are graphs for explaining the offset of thedistortion correction signal in a prior art as a reference case;

FIG. 13 is a graph for explaining the distortion correction signal inanother prior art as a reference case;

FIGS. 14a to 14 f are views for explaining the distortion correctingoperation in the embodiments of the present invention;

FIGS. 15a to 15 f are views for explaining the distortion correctingoperation in a prior art as a control;

FIGS. 16a and 16 b are diagrams for explaining the pincushiondistortion;

FIG. 17 is a block diagram showing the basic structure of a CRT monitorincluding the distortion correcting circuit;

FIG. 18 is a block diagram showing the configuration which is disclosedin JP-A-6-334887.

PREFERRED EMBODIMENTS OF THE INVENTION

The embodiments of the present invention are now described in the below.In the present invention, a partial area in which a current sweepingposition is located is detected by comparing an amplitude of a verticalsawtooth wave with one reference level or a plurality of referencelevels while a plurality of partial areas (such as an upper and lowerparts) are sectioned and located along a vertical sweeping direction onthe screen, and an amplitude of the distortion correction signal isindividually controlled for each of the partial areas on the screen byvariably controlling a gain of an amplifier circuit which adjusts theamplitude of a left and right pincushion distortion (termed sidepincushion distortion) correction signal (parabolic wave) for each ofthe partial areas of the screen on the basis of the detection result.The apparatus is configured so that the amplitude of the side pincushiondistortion correction signal is smoothly continued between the partialareas of the screen without any noticeable step. For example, theamplitude of the side pincushion distortion correction signal issmoothly continued at the center of the screen in case of partial areasbeing made up of the upper and lower areas of the screen.

Referring to FIG. 1, a distortion correcting circuit in accordance withan embodiment of the present invention comprises a comparator circuit 10which receives as an input a vertical sawtooth wave and compares anamplitude of the vertical sawtooth wave (the amplitude of the verticalsawtooth wave in a scanning period) with a reference level which is setto an amplitude of the vertical sawtooth wave corresponding to a centerposition of the screen along a vertical sweeping direction. If thevoltage amplitude of the vertical sawtooth wave are to be compared, thereference voltage Vref is provided to the comparator circuit 10 as thereference level. The distortion correcting circuit further comprises amultiplier circuit 11 which receives vertical sawtooth wave and thereference level and generates a parabolic wave of a difference betweenthe input vertical sawtooth wave and the reference level, a variablegain amplifier 12 which adjusts an output amplitude of the multipliercircuit 11 and a control circuit 13 which provides, based on thecomparison result by the comparator circuit 10, a first gain A1 to thevariable gain type amplifier circuit 12 when a comparison result by thecomparator circuit 10 indicates that the amplitude of the verticalsawtooth wave is lower than the reference level and a second gain A2 tothe variable gain type amplifier circuit 12 when a comparison result bythe comparator circuit 10 indicates that the amplitude of the verticalsawtooth wave is higher than or equal to the reference level. Adistortion correction signal Vo is output from an output terminal of theamplifier circuit 12.

Referring now to FIG. 2, a distortion correcting circuit in accordancewith another embodiment of the present invention comprises a comparatorcircuit 10 which compares an amplitude of the vertical sawtooth wavesignal with a reference level which is set to the amplitude of thevertical sawtooth wave corresponding to a center of the screen in avertical sweeping direction, a variable gain amplifier circuit 12 whichamplifies a difference between the vertical sawtooth wave and thereference level, a control circuit 13 which selects and provides, basedon the comparison result by the comparator circuit 10, a first gain tothe variable gain type amplifier 12 when the amplitude of the verticalsawtooth wave is lower than the reference level and selects and providesa second gain to the variable gain type amplifier 12 when the amplitudeof the vertical sawtooth wave is higher than or equal to the referencelevel, a multiplier circuit 11 which generates a parabolic wave from theoutput from the variable gain amplifier circuit 12 and the differencebetween the vertical sawtooth and the reference voltage and outputs itas a distortion correction signal Vo.

The embodiments of the present invention will be described in detailwith reference to drawings. FIG. 1 illustrates a schematic diagram of adistortion correcting circuit in accordance with a first embodiment ofthe present invention. Referring to FIG. 1, the distortion correctingcircuit of the first embodiment of the present invention comprises acomparator circuit 10 which receives as an input a vertical sawtoothwave Vsaw generated by the sawtooth wave generating circuit of thevertical output unit (not shown) and compares the voltage of thevertical sawtooth wave in a scanning period with a reference voltageVref which is set to the voltage of the vertical sawtooth wavecorresponding to a center position of the screen in a vertical sweepingdirection, a multiplier circuit 11 which receives the vertical sawtoothwave Vsaw and reference voltage Vref and generates a parabolic wave ofthe difference between the vertical sawtooth wave Vsaw and the referencevoltage Vref (Vsaw−Vref), a variable gain type amplifier circuit 12which adjusts an amplitude of the output of the multiplier circuit 11,and a control circuit 13 which receives a comparison result by thecomparator circuit 10 as a selection control signal and sets the gain ofthe amplifier circuit 12 to a first gain A1 when the voltage of thevertical sawtooth wave is lower than the reference voltage Vref, and toa second gain A2 when the voltage of the vertical sawtooth wave is equalto or higher than the reference voltage Vref, respectively. A distortioncorrection signal Vo having a period equal to one vertical period isoutput from the output terminal of the amplifier circuit 12. Theamplitude of the distortion correction signal Vo is adjusted by theamplifier circuit 12 with gains which are different at a first half anda latter half intervals of one vertical period.

The amplifier circuit 12 which adjusts the amplitude of the output ofthe multiplier circuit 11 is comprised of a voltage controlledamplifier, the gain of which is varied based on a control voltage whichis fed to the amplifier circuit 12. In this case, the control circuit 13selects a control voltage Vc1 corresponding to the first gain A1 and acontrol voltage Vc2 corresponding to the second gain A2 on the basis ofa comparison result by the comparator circuit 10 and provides theselected control voltage to the voltage controlled amplifier circuit 12.

The reference voltage Vref which is fed to the comparator circuit 10 isset to a voltage of the vertical sawtooth wave corresponding to thecenter position of the screen in a vertical sweeping direction. Thecomparator circuit 10 is made up of a voltage comparator. Alternatively,the amplitude of the vertical sawtooth wave may be supplied to thecomparator circuit 10 as an current and is converted to a voltage whichis compared with the reference voltage. Alternatively, the current maybe compared with the current corresponding to the reference voltage.

A bottom of a center of the distortion correction signal (parabolicwave) is located at a center position of the screen as shown in FIG. 10cas also in the case in which the center P of the screen is shifted fromthe center Q of the vertical sawtooth wave as shown in FIGS. 10a and 10b. This shift is generated, for example by variations in mounting ofdeflecting yoke in a manufacturing process. The distortion correctionsignal can be generated, which is capable of properly correcting theside pincushion distortion which is asymmetric in a vertical directionon the screen.

As for a reference case in which the reference voltage Vref is set tothe center of the amplitude of the vertical sawtooth wave, a shift of acenter of the screen from a center of the amplitude of the verticalsawtooth wave makes the distortion correction signal such one as shownby a dotted line in FIG. 10c and the bottom of the center of thedistortion correction signal (parabolic wave) is not located in thecenter position of the screen. Accordingly, the distortion which dependson the position on the screen can not be properly corrected. Such aproblem occurs in, for example, an apparatus which is disclosed inabove-mentioned JP-A-5-308538. Since the parabolic wave is not generatedby the multiplier circuit, but is generated by an integrator circuit inthe apparatus which is disclosed in JP-A-5-308538 as mentioned above,the center of the amplitude of the vertical sawtooth wave matches thebottom of the center of the distortion correction signal so that thedistortion correction signal is such one as shown by the dotted line inFIG. 10c even if the center of the vertical sawtooth wave is shiftedfrom the center of the screen. With an apparatus disclosed in theabove-mentioned JP-A-5-308538, in which the correction section in whichthe gain is changed is set with reference to the verticalsynchronization signal, if the center of the amplitude of the verticalsawtooth wave is shifted from the center of the screen in a verticaldirection after the adjustment of the distortion, the distortioncorrection will become ineffective and hence it is necessary to performa distortion adjustment again, for example, by changing the relationbetween the correction section and the correction amount. In contrast tothis, the distortion correction signal is generated by using thereference voltage which is set to a voltage corresponding to the centralposition of the screen in the present embodiment, so that the bottom ofthe center of the distortion correction signal constantly coincides withthe center of the screen. As a result, even if the center of thedisplayed image is shifted upwardly or downwardly on the screen afterthe distortion adjustment is conducted once, no readjustment of the sidepincushion distortion is necessitated.

A second embodiment of the present invention will now be described. FIG.2 is a diagram illustrating a configuration of a distortion correctingcircuit which forms the second embodiment of the present invention.Referring to FIG. 2, the distortion correcting circuit in accordancewith the second embodiment of the present invention is different fromthat of the first embodiment in the arrangement of the multipliercircuit and the variable gain type amplifier circuit. More specifically,the distortion correcting circuit of the second embodiment comprises acomparator circuit 10 which receives a vertical sweeping verticalsawtooth wave Vsaw as an input and compares the voltage of the verticalsawtooth wave Vsaw with a reference voltage Vref which is set to thevoltage of the vertical sawtooth wave corresponding to a center positionof the screen in a vertical sweeping direction, a variable gain typeamplifier circuit 12 which amplifies a difference voltage between thevoltage of the vertical sawtooth wave Vsaw and the reference voltageVref, a control circuit 13 which receives a comparison result by thecomparator circuit 10 as a selection control signal and sets a gain ofthe amplifier circuit 12 to a first gain A1 when the voltage of thevertical sawtooth Vsaw is lower then the reference voltage, and to asecond gain A2 when the voltage of the vertical sawtooth Vsaw is equalor higher than the reference voltage Vref, respectively, and amultiplier circuit 11 which generates a parabolic wave by using theoutput from the amplifier circuit 12 to output the parabolic wave as adistortion correction signal Vo. As is the case of the first embodiment,the amplifier circuit 12 may be constituted of a voltage controlledamplifier circuit (VCA) having a gain which is varied on the basis of acontrol voltage fed to the amplifier. In this case, the control circuit13 provides control voltages Vc1 and Vc2 corresponding to the first andsecond gains A1 and A2, respectively to the voltage controlled amplifiercircuit 12 based upon a comparison result in the comparator circuit 10.

The multiplier circuit 11 outputs the distortion correction signal Vohaving an amplitude corresponding to a value (A×(Vsaw−Vref)²) whichcorresponds to an output from the amplifier circuit 12 having a gain A(A×(Vsaw−Vref)) which is multiplied by the difference between thevertical sawtooth wave Vsaw and reference voltage Vref (Vsaw−Vref).

Any known circuit may be utilized as the multiplier circuit 11.Connections of signals in the multiplier circuit 11 as shown in FIG. 2will now be described with reference to a Gilbert multiplier circuitwhich outputs a value corresponding to a product of the input voltages Xand Y. FIG. 4 is a diagram of a basic configuration of the Gilbertmultiplier circuit. Referring to FIG. 4, this multiplier circuitcomprises a first differential pair of transistors Q1, Q2, a constantcurrent source I₀, and second and third differential pairs oftransistors Q3, Q4 and Q5, Q6. Emitters of transistors Q1 and Q2 arecommonly connected to the constant current source I₀, emitters oftransistors Q3 and Q4 are commonly connected to a collector of thetransistor Q1, and emitters of transistors Q5 and Q6 are commonlyconnected to a collector of the transistor Q2. Collectors of transistorsQ3 and Q4 and collectors of transistors Q5 and Q6 are mutuallycross-connected. A first voltage X is fed differentially to differentialinput terminals A+ and A− which are connected to the bases of the seconddifferential transistor pairs Q3, Q4 and third differential transistorpairs Q6 and Q5. A second voltage Y is fed differentially todifferential input terminals B+, B− which are connected to the bases ofthe first differential transistor pair Q1 and Q2. A current which isproportional to a value X×Y which is the first voltage X multiplied withthe second voltage Y is output as a differential current ΔI(=I₁−I₂)between the collector currents of the transistors Q3 and Q6. Input andoutput terminals of a current mirror current (not shown) are connectedto the collectors of the transistors Q3, Q6, respectively, so that adifferential current ΔI between the collector currents of thetransistors Q3 and Q6 are taken out from the connection points of theoutput terminal of the current mirror circuit and the collector of thetransistor Q6. This differential current ΔI is used if the multipliercircuit 11 outputs the distortion correction signal (parabolic wave). Avoltage corresponding to a value X×Y is output by converting thedifferential current ΔI to a voltage by a current-voltage convertercircuit (not shown) if the multiplier circuit 11 outputs the distortioncorrection signal Vo as a voltage.

In the present embodiment, the outputs (differential outputs) of theamplifier circuit 12 are connected to the differential terminals B+, B−of the multiplier circuit 11, the vertical sawtooth wave Vsaw is fed tothe input terminals A+ and the reference voltage Vref is fed to theinput terminal A−. In this case, the distortion correcting signal Vohaving an amplitude corresponding to a value {A×(Vsaw−Vref)}×(Vsaw−Vref)is output from the multiplier circuit 11.

In this embodiment, as is the case with the first embodiment, even ifthe center of the screen is shifted from the center of the verticalsawtooth wave, the bottom of the center of the distortion correctionsignal (parabolic wave) is located at the center of the screen, andhence the distortion correction signal which is capable of properlycorrecting the side pincushion distortion which is asymmetric withrespect to upper and lower areas of the screen.

FIG. 3 is a diagram showing a modification of the second embodiment ofthe present invention. Referring to FIG. 3, this modification isdifferent from the above mentioned second embodiment in that a signalfed to the multiplier circuit 11 is only an output signal A×(Vsaw−Vref)from the amplifier circuit 12. The distortion correction signal Vo isoutput having an amplitude corresponding to a value{A×(Vsaw−Vref)}×{A×(Vsaw−Vref)} from the multiplier circuit 11.

In the modification, outputs (differential outputs) of the amplifiercircuit 12 is connected to differential input terminals B+, B− of theamplifier circuit 11 as shown in FIG. 4 and is also connected todifferential input terminals A+, A−, so that A²×(Vsaw−Vref)² is outputas the distortion correction signal Vo. The operation and effect of thismodification is identical with that of the second embodiment.

A third embodiment of the present invention will be described. FIG. 5 isa diagram illustrating the configuration of the distortion correctingcircuit which is the third embodiment of the invention. Referring now toFIG. 5, the distortion correcting circuit in accordance with the thirdembodiment of the present invention comprises a comparator circuit 10Awhich compares the voltage of the vertical sawtooth wave Vsaw with aplurality of reference voltages Vref1, Vref2, and Vref3 which are set tothe amplitudes of the vertical sawtooth waves respectively correspondingto predetermined vertical positions (predetermined positions locatedalong a vertical sweeping direction) on the screen, multiplier circuit11 which generates a parabolic wave from the vertical sawtooth wave, avariable gain type amplifier circuit 12 which adjusts the output voltageof the multiplier circuit 11 and a control circuit 13.

The control circuit 13A, based on a detection result by the comparatorcircuit 10A what section the voltage of the vertical sawtooth wave Vsawis located at with regard to a plurality of sections of a verticalperiod, in which these sections are defined by the plurality ofreference voltages Vref1, Vref2, and Vref3, selects a gain correspondingto the section from a plurality of gains A1 to A4 and provides theselected gain to the variable gain type amplifier circuit 12. Thevariable gain type amplifier circuit 12 amplifies the output voltagefrom the multiplier circuit 11 with the gain and outputs the amplifiedsignal from an output terminal as a distortion correction signal.

The multiplier circuit 11 is constituted to multiply the differencebetween the vertical sawtooth wave Vsaw and the reference voltage Vref.The reference voltage Vref which is fed to the multiplier circuit 11 maybe fixed to, for example, the reference voltage Vref 2, or may beselected among the reference voltages Vref1, Vref2, and Vref3 dependingupon which section of the vertical period the voltage of the verticalsawtooth wave Vsaw is located at and may be supplied from the comparatorcircuit 10A to the multiplier circuit 11.

Referring to FIG. 5, the reference voltages Vref1, Vref2, and Vref3which are fed to the comparator circuit 10A are generated by dividingthe constant voltages Vmax, and Vmin by voltage dividing resistors R1,R2, R3,and R4 in FIG. 5. The voltages Vref1, Vref2, and Vref3 may begenerated respectively by discrete bias voltage sources. The number ofthe reference voltages may be 2, or more than 3. The control circuitwhich receives output (plural bits outputs) from the comparator circuit10A and selects a gain among the plurality of gains A1 to A4 is made upof, for example, a decoder circuit and a change-over switch (both notshown). The decoder circuit decodes an output result from the comparatorcircuit 10A. The change-over switch selects one of the plurality ofgains A1 to A4 at a time depending upon which of the sections divided bythe reference voltages Vref1, Vref2, and Vref3, the voltage of thevertical sawtooth wave Vsaw received is currently located at based upona decoding result from the decoder and outputs the selected gain to theamplifier circuit 12 as the gain A.

In the third embodiment of the present invention, the reference voltageVref2 is for example set to the voltage of the vertical sawtooth wavecorresponding to the center of the screen in a vertical sweepingdirection. In FIG. 5, the voltage dividing resistors R1, R2, R3, and R3for generating the reference voltages are comprised of variableresistors, the resistances of which are variable and the resistances ofthe resistors are adjusted on an adjustment process for shipment in amanufacturing process of the display apparatus so that they have desiredresistances. Alternatively, the preset values of the reference voltagesmay be stored in a non-volatile memory such as EEPROM (not shown). Thevalues of the non-volatile memory may be set into a register (not shown)on a power on sequence, so that the reference voltages are output bysetting the values of the reference voltage source (not shown) basedupon the values in the register.

In this embodiment, as is the case with the first and second embodiment,even if the center of the screen is shifted from the center of thevertical sawtooth wave, the bottom of the center of the distortioncorrection signal (parabolic wave) is located at the center of thescreen, and hence the distortion correction signal which is capable ofproperly correcting the side pincushion distortion which is asymmetricwith respect to upper and lower areas of the screen. Since the thirdembodiment is configured in such a manner that the amplitude of thedistortion correction signal is adjusted by a gain corresponding to eachof the plurality of sections of the screen in a vertical directionthereof, the side pincushion distortion can be properly adjusted foreach section.

Now, a fourth embodiment of the present invention will be described.FIG. 6 is a diagram illustrating the configuration of the distortioncorrecting circuit which forms the fourth embodiment of the presentinvention. Referring now to FIG. 6, the distortion correcting circuit issubstantially identical with that in the third embodiment except thatthe arrangement of the amplifier circuit 12 and the multiplier circuit11 is changed so that the distortion correction signal Vo is output fromthe multiplier circuit 11 which multiplies an output signal amplified bythe amplifier circuit 12. The output of the amplifier circuit 12(A×(Vsaw−Vref)) and, the vertical sawtooth wave Vsaw and referencevoltage Vref are fed to the multiplier circuit 11 as is the case withthe second embodiment, and A×(Vsaw−Vref)² is output as the distortioncorrection signal Vo.

FIG. 7 is a circuit diagram showing the detailed configuration of thecontrol circuit 13 which provides a control voltage to the amplifiercircuit 12 made up of a voltage controlled amplifier circuit in thefirst embodiment of the present invention.

Referring now to FIG. 7, the control circuit 13 comprises a first andsecond registers 14 and 15 which store digital values corresponding to afirst and second control voltages, respectively, an AND circuit 16, anAND circuit 17, 2n-input and n-output OR circuit 18, and a digital toanalog converter (DAC) 19. The AND circuit 16 outputs in parallel n-bitsof signals, each of which is a result of logical AND operation of eachbit of the output (n bits) of the first register 14 and a comparisonresult signal from a comparator circuit 10. The AND circuit 16 outputsthe output signal from the first register 14 and 0 when the comparisonresult output from the comparator circuit 10 is 1 and 0, respectively.The AND circuit 17 outputs in parallel n-bits of signals, each of whichis a result of logical AND operation of each bit output from the secondregister 15 (n bits) and a complimentary signal of a comparison resultfrom the comparator circuit 10. The AND circuit 17 outputs the outputsignal from the second register 15 and 0 when the comparison resultoutput from the comparator circuit 10 is 0 and 1, respectively. The2n-input and n-output OR circuit 18 receives n-bit outputs from the ANDcircuit 16, 17 and outputs in parallel n bits of signals, each of whichis a result of a logical OR operation of corresponding pair of bitsignals. The output signal from the OR circuit 18 is fed to the digitalto analog converter (DAC) 19 and the analog voltage output from thedigital to analog converter 19 is fed to the voltage controlledamplifier circuit 12 as a control signal.

The configuration which is shown in FIG. 7 is equivalent to the functionof a selector which comprising n pairs of first and second tri-statebuffers (when one of the buffers is output enabled, the other buffer isset in a high impedance state) to which receives corresponding two bitsfrom first and second registers 14 and 15. The output enable terminalsof the first and second buffers are controlled by the output and theinverted output of the comparator circuit 10, respectively. The outputterminals of the first and second buffers are connected and theconnection node of the output terminals of the first and second buffersis connected to the corresponding input of the digital to analogconverter 19.

On an inspection or test process for shipment of the display apparatusin a manufacturing process, the values of the gains to be set in thefirst and second registers 14, 15 are written in a non-volatile memorysuch as EEPROM (electrically-erasable and programmable read-only memory)provided in the display apparatus. On an initialization process executedat the time of power on sequence of the display apparatus, the values ofthe gains are read from the non-volatile memory by a CPU (not shown) andare set into the first and second registers 14, 15.

FIG. 8 is a diagram showing in detail the configuration of the controlcircuit for supplying a control voltage to the amplifier circuit whichis made up of a voltage controlled amplifier circuit in a modificationof the above-mentioned second embodiment. Referring now to FIG. 8, theconfiguration of the control circuit 13 is similar to the controlcircuit 13 which is shown in FIG. 7.

FIGS. 9 and 10 are schematic views for explaining the principle ofoperation in the above mentioned embodiments of the present invention.The relation between a raster image displayed on a CRT (FIG. 9a), avertical sawtooth wave (FIG. 9b) and a distortion correction signal(FIG. 9c) when a center of the amplitude of the vertical sawtooth wavecoincides with a center of the screen in a vertical sweeping directionis shown in FIG. 9. The waveforms of the vertical sawtooth wave, and theoutput of the comparator circuit 10, the output of the amplifier circuit12 and the output of the multiplier circuit 11 (distortion correctingsignal) in the circuit shown in FIG. 8 are shown in FIG. 9d. Therelation between the levels of the vertical sawtooth wave and thereference voltage Vref is reversed (the output from the comparatorcircuit 10 is reversed) at the center of the amplitude of the verticalsawtooth wave, so that the output of distortion correction signal of themultiplier circuit 11 which multiplies the output of the amplifiercircuit 12 becomes symmetric with respect to the center of the screen asshown in FIG. 9c.

The relation between a raster image displayed on the CRT (FIG. 10a), thevertical sawtooth wave (FIG. 10b) and the distortion correction signal(FIG. 10c) when the center of the amplitude of the vertical sawtoothwave does not coincides with the center of the screen in a verticalsweeping direction and is shifted therefrom in the configuration of FIG.8 is shown in FIG. 10. The waveforms of the vertical sawtooth wave, andthe output of the comparator circuit 10, the output of the amplifiercircuit 12 (output of VCA 12) and the output of the multiplier circuit11 (distortion correction signal) in the circuit shown in FIG. 8 areshown in FIG. 10d. If the center of the amplitude of the verticalsawtooth is shifted from the center of the screen which is a verticalsweeping direction, for example, the upper end of a raster (starting endof the scanning) is shifted downwardly and a ratio of a time intervalbetween the raster upper end (scanning starting end) and the screencenter P to a time interval between the screen center P and the rasterlower end (scanning completion end) is r:s (r<s in case shown in FIG.10), the relation between the waveform Vsaw of the vertical sawtoothwave fed to the comparator circuit 10 and the reference voltage Vrefwill become as shown in FIG. 10d. The reference voltage Vref is set tothe voltage (voltage of the vertical sawtooth wave) corresponding to thescreen center P in a vertical scanning direction. In this case, thereference voltage is lower than the voltage of the amplitude center Q ofthe input vertical sawtooth. On the contrary, if the shift between theamplitude center of the vertical sawtooth and the screen center in avertical sweeping direction is reversed to that shown in FIGS. 10a and10 b (r>s in FIG. 10(d)), the reference voltage Vref is higher than thevoltage of the amplitude center Q of the input vertical sawtooth wave.

The amplifier circuit 12 amplifies the input signal at a first gain A1until the voltage of the vertical sawtooth wave exceeds the referencevoltage Vref after the voltage Vsaw increases from the scan startvoltage. The output signal of the comparator circuit 10 becomes reversedwhen the vertical sawtooth wave exceeds the reference voltage Vref.Thereafter, the amplifier circuit 12 amplifies the input signal at thesecond gain A2 until the completion of scanning, and hence thedistortion correction signal Vo becomes a parabolic wave having itsbottom located at the screen center P.

The reference voltage Vref is set up to a value corresponding to thescreen center by the adjustment for example on a final test for ashipment in a manufacturing process of the display apparatus. The set upvalue of the reference voltage may be written in a non-volatile memorysuch as EEPROM, and on a power on sequence, the set up value of thereference voltage is set in a register (not shown), from which the valueis fed to a digital/analog converter (not shown) which outputs areference voltage.

FIG. 11a is a graph illustrating the relation between the distortioncorrection signal Vo (the amplitude in one vertical period) and thevertical sawtooth wave (solid line connecting rhombus marks) in theforegoing first and second embodiments. FIG. 11b is a graph illustratingan enlarged central area in FIG. 11b in detail (an axis ordinaterepresented in unit of 100 mV). Since the multiplication is notindividually performed in upper and lower areas of sweeping in thepresent invention, no step occurs in the parabolic signalVpara=K×(Vsaw+Voff1)² from the multiplier circuit 11 between the upperand lower areas of the screen even if there is an input offset Voff1 ofthe multiplier circuit 11.

With respect to an input offset of the amplifier circuit 12, thedistortion correction signal Vo output from the amplifier circuit 12becomes Vo=A×(Vpara+Voff2) (wherein A denotes the gain of the amplifiercircuit 12). The gain A is set to different values at first and latterhalves of the vertical period. The distortion correction signal Vowithout offset is represented by a dotted line (no Voff) connectingsquare marks in FIG. 11. The distortion correction signal Vo with offsetis represented by a line (with Voff) connecting marks X.

It is assumed that the vertical sawtooth wave has an amplitude of ±5V,the input offset Voff1 of the multiplier circuit 11 is −100 mV, and theinput offset of the amplifier circuit 12 is 100 mV, both the gains A1,A2 in the first and latter halves of one vertical interval are 1, thestep at the screen center in the distortion correcting signal V (withVoff) is hardly noticed as shown in FIG. 11.

As a reference case, the configuration corresponding to that shown inFIG. 18 in which a slice point is set to the screen center will beexplained. FIG. 12a is a graph showing a relation between the distortioncorrecting signal Vo (the amplitude of one vertical period) comprising aparabolic wave and the vertical sawtooth wave (a solid line connectingrhombic marks) in the configuration corresponding to that shown in FIG.18 (provided that the slice point is located at the screen center). FIG.12b is a graph showing the enlarged central area in the screen in FIG.12a (an axis of ordinate represented in unit of 100 mV).

If there should be variations between the output offsets of twodifferential comparator circuit (slice circuits), they are squared bythe multiplier circuit. Taking into account the output offset Voff1 ofthe differential comparator circuit (slice circuit), the output of themultiplier circuit will become Vpara=K×(Vsaw+Voff1)². If the inputoffsets of the amplitude circuits (gain control circuits) 107, 108 aredenoted as Voff2, the distortion signal which is a sum of the outputs oftwo amplitude adjusting circuits 107 from an adder 109 becomesVo=A×(Vpara+Voff2). If the distortion correction signal Vo is slicedinto two parts corresponding to upper and lower areas of the screen bytwo differential comparator circuits, the signal Vo=A×(Vpara+Voff2) isreplaced with values of the differential comparator circuit andamplitude adjusting circuit in the first and latter halves of onevertical period, respectively.

If it is assumed that for the vertical sawtooth wave of ±5V, the firstdifferential comparator circuit (slice circuit) 102 in FIG. 18 has anoffset of −100 mV, the second differential comparator circuit (slicecircuit) 103 has no offset, the first amplitude adjusting circuit 107has an input offset of 100 mV, the second amplitude adjusting circuit108 has an input offset of 0V, the first and second amplitude adjustingcircuits 107, 108 have a gain of 1, the distortion correcting signalwill have an step of 100 mV in the center thereof as shown in FIG. 12(refer to FIG. 12b). This step is changed by the gain set up to theamplitude adjusting circuit. A step in a horizontal direction occurs inthe raster image as shown in FIG. 15c when a step occurs in thedistortion correction signal.

In contrast to this, no step appears due to an offset in accordance withthe present invention, so that a side pincushion distortion can beproperly corrected as shown in FIG. 14.

The characteristics of the distortion correction signal will beconsidered with reference to an apparatus as set forth inJP-A-11-313222. In this apparatus, a parabolic wave (=k1×Vsaw²) and athird power wave (A×K2×Vpara×Vsin) are used. The distortion correctionsignal Vo becomes as shown in FIG. 13. If only the lower area of thescreen is corrected, it suffices to make the distortion correctionsignal for the upper area of the screen 0, but a distortion newly occursin the vicinity of ¼ and ¾ of the screen. In other words, when a thirdpower wave which is symmetric with respect to the center of the screenin a vertical direction and correction of, for example, upper area ofthe screen is conducted, the third power wave gives an influence uponthe parabolic wave in the lower area of the screen if the distortioncorrecting wave is zero at the upper area of the screen.

In contrast to this, no combination of a parabolic wave with a thirdpower wave is used and the distortion correction is simplified inaccordance with the present invention. The distortion due to the thirdpower wave as in the configuration as set forth in JP-A-11-313222 willnot appear. Accordingly, the adjustment for the distortion is maderemarkably easy.

The distortion correcting circuit of the present invention may beapplied to any display apparatus including a CRT and is used as thedeflection distortion correcting circuit 108 for the display apparatusshown in FIG. 17. The distortion correction signal having one periodwhich is one vertical period is amplitude-modulated with a horizontaldeflection current and is supplied from the horizontal deflection outputcircuit 210 to a horizontal deflection coil.

The distortion correcting circuit of the above-mentioned embodiments ispreferably applied to the correction of the vertically asymmetricdistortion of the display apparatuses which increases their size andflatness of the screen. In comparison with the configuration having twosystems each comprising a differential comparator circuit, multipliercircuit, amplitude adjusting circuit (voltage controlled amplifiercircuit) for each of upper and lower areas of the screen like theconfiguration disclosed in the above-mentioned JP-A-6-334887 (refer toFIG. 8), only one system comprising a differential comparator circuit,multiplier circuit, and amplitude adjusting circuit is required inaccordance with the present invention. The circuit configuration of thecontrol circuit is small in scale as shown in FIGS. 7 and 8. The offsetin the present invention is remarkably smaller in comparison with thatof the configuration disclosed in JP-A-6-334887, so that no step appearsin the raster image. Reduction in cost and improvement in image qualitycan be achieved by a simple circuit configuration.

Although the present invention has been described with reference toembodiments, the present invention is not limited to the foregoingembodiments. It is to be understood that the present invention includesany modifications and change made by those skilled in the art withoutdeparture from the spirit and scope of the invention as set forth in theclaims.

The meritorious effects of the present invention are summarized asfollows.

As mentioned above, the present invention provides a meritorious effectthat the distortion of image can be individually corrected at each areassuch as in the upper and lower areas of the screen while reducing thescale of the circuit. The reason is that in the present invention, onlyone system comprising a multiplier circuit for generating a distortioncorrecting parabolic wave and an amplifier circuit is required for aplurality of regions which divide the screen.

The present invention further provides a meritorious effect that a sidepincushion distortion and a vertically asymmetric distortion such as atrapezoidal distortion can be properly corrected even if a center of avertical sawtooth wave is shifted from a center of the screen, while theshift for example being caused by variations in mounting position of anelectron gun. The reason is that in the present invention, the amplitudeof the vertical sawtooth wave is compared with the reference voltagewhich is set to an amplitude of the vertical sawtooth wave correspondingto the center of the screen for changing the gain of the amplifiercircuit at upper and lower areas of the screen.

In accordance with the present invention, the distortion adjustment ofthe display apparatus is simplified by a simple circuit configuration,and reduction in manufacturing cost can be achieved. The presentinvention is preferably applied to the correction of verticallyasymmetric distortion of the display apparatus having a large sizescreen.

The present invention remarkably reduces an offset at the central partof the distortion correction signal (between partial areas), while theoffset being caused for example by offsets of a multiplier circuit andan amplifier circuit for amplitude adjustment, so that a step isprevented from appearing on the raster image and contributes to animprovement in image quality.

In accordance with the present invention, the distortion correctionsignal is generated by using a reference voltage which is set to avoltage corresponding to the center position on the screen, so that thebottom of the center of the distortion correction signal (parabolicwave) constantly coincides the center position of the screen.Accordingly, necessity of readjustment for the side pincushiondistortion is unnecessary, and an adjustment process becomes remarkablyeasy even if the center of the amplitude of the vertical sawtooth waveis shifted from the center of the screen after the adjustment fordistortion has been conducted once.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

What is claimed is:
 1. A distortion correcting circuit comprising:comparing means for comparing an amplitude of a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) with onepredetermined reference level or a plurality of predetermined referencelevels to detect which part of a screen in a vertical sweeping directiona currently sweeping position is located; amplifying means for adjustingan amplitude of a side pincushion distortion correction signal generatedfrom said vertical sawtooth wave; and control means for individuallysetting a gain for each of partial regions of the screen to saidamplifying means on the basis of a detection result by said comparingmeans.
 2. A distortion correcting circuit comprising: comparing meansfor comparing an amplitude of a sawtooth wave signal used for a verticalsweep (termed “vertical sawtooth wave”) with a predetermined referencelevel to detect which of an upper and lower areas of a screen acurrently sweeping position is located at; and amplifying means foradjusting an amplitude of a side pincushion distortion correction signalgenerated from said vertical sawtooth wave; and control means forindividually setting a gain for each of an upper and lower areas of thescreen to said amplifying means on the basis of a detection result bysaid comparing means.
 3. A distortion correcting circuit comprising: acomparator circuit which receives a sawtooth wave signal used for avertical sweep (termed “vertical sawtooth wave”) and compares anamplitude of said vertical sawtooth wave in a scanning period with onereference level or a plurality of different reference levels, each ofwhich is set respectively to an amplitude value of the vertical sawtoothwave corresponding to each predetermined position of the screen in avertical sweeping direction; a variable gain type amplifier circuitwhich adjusts an amplitude of an output signal from a multiplier circuitfor generating a parabolic wave; and a control circuit whichalternatively selects one of a plurality of gains to provide theselected gain to said variable gain type amplifier circuit on the basisof a comparison result by said comparator circuit depending upon whichof regions divided by said reference level the amplitude of saidvertical sawtooth wave is located, wherein a distortion correctionsignal is output from said variable gain type amplifier circuit.
 4. Adistortion correcting circuit comprising: a comparator circuit whichreceives a sawtooth wave signal used for a vertical sweep (termed“vertical sawtooth wave”) and compares a voltage of said verticalsawtooth wave in a scanning period with a reference voltage which is setto an amplitude of the vertical sawtooth wave corresponding to a centerof a screen in a vertical sweeping direction; a multiplier circuit whichreceives said vertical sawtooth wave and said reference voltage asinputs and generates a parabolic wave of a difference between thevoltage of said vertical sawtooth wave and said reference voltage; avariable gain type amplifier circuit which adjusts an amplitude of anoutput signal output from said multiplier circuit; and a control circuitwhich provides, on the basis of a comparison result by said comparatorcircuit, a first and second predetermined gains to said variable gaintype amplifier circuit when the voltage of said vertical sawtooth wavesignal is lower, and equal to or higher than said reference voltage,respectively, wherein a distortion correction signal is output from saidvariable gain type amplifier circuit.
 5. A distortion correcting circuitcomprising: a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesa voltage of said vertical sawtooth wave in a scanning period with areference voltage which is set to an amplitude of the vertical sawtoothwave corresponding to a center of a screen in a vertical sweepingdirection; a multiplier circuit which generates a parabolic wave of adifference between the voltage of said vertical sawtooth wave and saidreference voltage; a voltage controlled amplifier circuit which receivesa control voltage, a gain of said amplifier circuit being changed bysaid control voltage, and said parabolic wave to amplify the voltage ofthe said parabolic wave at the gain specified by the control voltage andoutput the amplified parabolic wave as a distortion correction signal;and a control circuit which provides, on the basis of a comparisonresult by said comparator circuit, a first predetermined control voltageto said variable gain type amplifier circuit when the voltage of saidvertical sawtooth wave signal is lower and provides a secondpredetermined control voltage to said variable gain type amplifiercircuit when the voltage of said vertical sawtooth wave signal is equalto or higher than said reference voltage.
 6. A distortion correctingdisplay device comprising: a display apparatus wherein a horizontaldeflection coil provided in said display apparatus is driven with ahorizontal deflection current, which is amplitude-modulated with adistortion correction signal having a vertical interval period outputfrom a distortion correcting circuit, said distortion correcting circuitcomprising a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesthe voltage of said vertical sawtooth wave in a scanning period with areference level or a plurality of reference levels, each of which is setto an amplitude of the vertical sawtooth wave corresponding to apredetermined vertical position of a screen; a variable gain typeamplifier circuit which amplifies said vertical sawtooth wave; a controlcircuit which selects one of plurality of predetermined gains dependingupon which of regions divided by said reference level the amplitude ofsaid vertical sawtooth wave is located on the basis of a comparisonresult by said comparator circuit and provides the gain selected to thevariable gain type amplifier circuit; and a multiplier circuit whichgenerates a parabolic wave from an output signal output from saidvariable gain type amplifier circuit to output said parabolic wave as adistortion correction signal.
 7. A distortion correcting circuitcomprising: a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesthe voltage of said vertical sawtooth wave in a scanning period with areference voltage which is set to an amplitude of the vertical sawtoothwave corresponding to a central position of a screen in a verticalsweeping direction; a variable gain type amplifier circuit whichreceives the sawtooth wave and the reference voltage and amplifies adifference between the sawtooth wave and the reference voltage; acontrol circuit which provides, on the basis of a comparison result bysaid comparator circuit, a first predetermined control voltage to saidvariable gain type amplifier circuit when the voltage of said verticalsawtooth wave signal is lower and provides a second predeterminedcontrol voltage to said variable gain type amplifier circuit when thevoltage of said vertical sawtooth wave signal is equal to or higher thansaid reference voltage; and a multiplier circuit which receives anoutput signal from said variable gain type amplifier circuit andgenerates a parabolic wave from the output signal output from saidvariable gain type amplifier circuit to output the parabolic wave as adistortion correction signal.
 8. A distortion correcting circuitcomprising: a comparator circuit which receives a sawtooth wave signalused for a vertical sweep (termed “vertical sawtooth wave”) and comparesthe voltage of said vertical sawtooth wave in a scanning period with areference voltage which is set to an amplitude of the vertical sawtoothwave corresponding to a central position of a screen in a verticalsweeping direction; a variable gain type amplifier circuit whichreceives the sawtooth wave and the reference voltage and amplifies adifference between the sawtooth wave and the reference voltage; acontrol circuit which provides, on the basis of a comparison result bysaid comparator circuit, a first predetermined control voltage to saidvariable gain type amplifier circuit when the voltage of said verticalsawtooth wave signal is lower and provides a second predeterminedcontrol voltage to said variable gain type amplifier circuit when thevoltage of said vertical sawtooth wave signal is equal to or higher thansaid reference voltage; and a multiplier circuit which receives anoutput signal from said variable gain type amplifier circuit andgenerates a parabolic wave from the output signal output from saidvariable gain type amplifier circuit to output the parabolic wave as adistortion correction signal.
 9. The distortion correcting circuit asdefined in claim 5 comprising: a first and second registers for storingtherein digital values corresponding to said first and second controlvoltages; a selector to which receives the outputs from said first andsecond registers and a signal indicating the comparison result by saidcomparator circuit as a selection control signal, selects and outputsoutput values of the first registers when said comparison result islower than said reference voltage and selects and outputs output valuesof the second registers when said comparison result is equal to orhigher than said reference voltage; and a digital to analog converterwhich receives the output signal from said selector and converts theoutput signal to an analog signal to output said analog signal, whereinthe output of said digital to analog converter is fed to said voltagecontrolled amplifier circuit as a control voltage.
 10. The distortioncorrecting circuit as defined in claim 8 comprising: a first and secondregisters for storing therein digital values corresponding to said firstand second control voltages; a selector to which receives the outputsfrom said first and second registers and a signal indicating thecomparison result by said comparator circuit as a selection controlsignal, selects and outputs output values of the first registers whensaid comparison result is lower than said reference voltage and selectsand outputs output values of the second registers when said comparisonresult is equal to or higher than said reference voltage; and a digitalto analog converter which receives the output signal from said selectorand converts the output signal to an analog signal to output said analogsignal, wherein the output of said digital to analog converter is fed tosaid voltage controlled amplifier circuit as a control voltage.
 11. Thedistortion correcting circuit as defined in claim 9 wherein saidselector comprising: a first logical AND circuit which receives anoutput of said first register and a signal indicating the comparisonresult by said comparator circuit, and outputs a logical AND of eachoutput bit of said first register and said signal indicating thecomparison result; a second logical AND circuit which receives an outputof said second register and a signal which is obtained by inverting thecomparison result signal output from said comparator circuit by aninverter, and outputs a logical AND of each output bit of said secondregister and said reversed comparison result signal; and a logical ORcircuit which receives an output of said first logical AND circuit andan output of said second logical AND circuit and outputs a signalrepresenting a logical OR of the output bit of said first logical ANDcircuit and an output bit of said second logical AND circuitcorresponding to said output bit of said first logical AND bit.
 12. Thedistortion correcting circuit as defined in claim 10 wherein saidselector comprising: a first logical AND circuit which receives anoutput of said first register and a signal indicating the comparisonresult by said comparator circuit, and outputs a logical AND of eachoutput bit of said first register and said signal indicating thecomparison result; a second logical AND circuit which receives an outputof said second register and a signal which is obtained by inverting thecomparison result signal output from said comparator circuit by aninverter, and outputs a logical AND of each output bit of said secondregister and said reversed comparison result signal; and a logical ORcircuit which receives an output of said first logical AND circuit andan output of said second logical AND circuit and outputs a signalrepresenting a logical OR of the output bit of said first logical ANDcircuit and an output bit of said second logical AND circuitcorresponding to said output bit of said first logical AND bit.
 13. Thedistortion correcting circuit as defined in claim 7 wherein saidmultiplier circuit generates said parabolic wave from the differencebetween said vertical sawtooth wave and said reference voltage, and asignal output from said variable gain type amplifier circuit.
 14. Thedistortion correcting circuit as defined in claim 8 wherein saidmultiplier circuit generates said parabolic wave from the differencebetween said vertical sawtooth wave and said reference voltage, and asignal output from said voltage controlled amplifier circuit.
 15. Adisplay apparatus comprising a distortion correcting circuit saiddistortion correcting circuit comprising comparing means for comparingan amplitude of a sawtooth wave signal used for a vertical sweep (termed“vertical sawtooth wave”) with one predetermined reference level or aplurality of predetermined reference levels to detect which part of ascreen in a vertical sweeping direction a currently sweeping position islocated; amplifying means for adjusting an amplitude of a sidepincushion distortion correction signal generated from said verticalsawtooth wave; and control means for individually setting a gain foreach of partial regions of the screen to said amplifying means on thebasis of a detection result by said comparing means, wherein ahorizontal deflection coil provided in said display apparatus is drivenwith a horizontal deflection current, which is amplitude-modulated witha distortion correction signal having a vertical interval period outputfrom said distortion correcting circuit.
 16. A display apparatuscomprising the distortion correcting circuit as defined in claim 9 and anon-volatile storage device, wherein values to be set in said first andsecond registers, which are obtained on an adjustment step in amanufacturing process are stored in said non-volatile storage device;and wherein each value which is stored in said non-volatile storagedevice is read-out on turning on of said apparatus and is stored in saidfirst and second registers.